Over the past week, military tensions on the Korean Peninsula have been building. Those tensions are fed by a stream of threats made by North Korea against the South and against the United States. Yesterday the hard-line regime in Pyongyang announced it was re-starting its nuclear program. And today another move has come.

NPR's Louisa Lim is here to tell us about it. She's in Beijing. Hi, Louisa.

LOUISA LIM, BYLINE: Good morning, Steve.

INSKEEP: So what has the North done now?

LIM: So this morning, like every other morning, scores of South Korean workers gathered at a border post to try to enter into North Korea, to work at this huge joint industrial zone six miles inside the North. And first they were delayed for several hours. Then North Korea announced that it wasn't allowing any more South Korean workers to enter the zone. So this is a big change.

There are still more than 800 South Koreans inside that zone in North Korea and they are being allowed to leave, if they should want to do so. But those who are outside are not being allowed into the joint industrial zone anymore.

INSKEEP: Let's remember this was an effort to advance relations between the two Koreas through economic activity. These are, what, factories run by South Koreans but on North Korean soil?

LIM: That's right. This is a joint industrial zone and there's about 123 South Korean companies that have factories there. There's around 800 South Koreans who manage those factories, and they're the ones who are not being allowed back in today. And they oversee about 53,000 North Korean workers whose salaries go to the North Korean government, so this generates a lot of money for Pyongyang - about $2 billion worth of trade a year. So it is an important source of income for Pyongyang.

So analysts say that messing with that could be going against Pyongyang's interest.

INSKEEP: OK, but help me understand the North Korean perspective here. What would their reason be for cutting off their own income stream because they're angry at the outside world?

LIM: Well, this is the kind of move by the North Koreans that we have actually seen before. In 2009 exactly the same thing happened. Once again it was in protest at the joint military drills carried out by the U.S. and South Korea. And in that case, this project, the border post was closed three times, and one of those times it was closed for three days. So there is a possibility that what we're seeing could be a short-term move, not an irreversible step.

INSKEEP: Well, let me back away from this situation with the industrial zone to the broader series of announcements and threats that North Korea has made in recent days. These kinds of things do happen so often that you can tend to discount them. But I'm wondering, Louisa, from where you sit, if things feel different this time.

LIM: Well, yes. I mean from one standpoint you are absolutely right. The North Koreans are experts in this sort of cycle of threats. And it often happens at this time of year when the joint military drills are being carried out, because North Korea sees those drills by the U.S. and South Korea as provocative as a rehearsal for invasion.

I think this time maybe the context is slightly different. It comes against the backdrop of other North Korean moves - the long-range rocket launch in December, a third military test in February. And so we've seen an escalation of tensions with the U.S. and South Korea really using different hardware, bringing in B-52 bombers, B-2 stealth bombers, even bringing in over F-22 jets. And all of those have heightened North Korean alarm.

And I think the other thing that's different is the fact that this year, North Korea's leader is Kim Jong-un. He's very young. He's relatively new in the job. Some analysts are saying this could be an internal power play by him. But the fact that he is inexperienced at brinksmanship does mean there's a higher risk of miscalculation.

INSKEEP: Louisa, has China - North Korea's only ally in the world - found anything to do to reign North Korea in?

LIM: Well, China is increasingly angered by North Korea's behavior and it has been for months. But it's finding it very difficult to calibrate the right kind of response. Today, China has called in ambassadors from the U.S. as well as from both Koreas to express serious concern about the situation. Beijing did also back the U.N. sanctions but they aren't taking a very tough line in public, and they think the reason for that is that Beijing's top priority, above all else, is stability.